Radiation in your own home?

Coming in to work at the nuclear plant one morning, Mr. Watras triggered the radiation monitor when he went into the plant !!! The monitor was of course place there to check workers for radioactive contamination when they left work! 监控器当然放在那里，用来检查工人在下班时是否受到放射性污染！ The radon level in the Watras home was measured to be 2500 picocuries per liter! 瓦特拉斯家中的氡电平被测得为每升2500皮居里！

What is radon? And why are they saying all those bad things about it?

Radon is a colorless, odorless gas, a radioactive byproduct of radium. It is part of the natural radioactive decay series starting with uranium-238. It is radioactive with a half-life of 3.8 days, decaying by the emission of alpha particles to polonium, bismuth, and lead in successive steps. 氡是一种无色、无味的气体，是镭的放射性衰变系列中的一个放射性副产物。它属于以铀-238开始的自然放射性衰变系列。氡具有3.8天的半衰期，通过发射α粒子衰变，依次转化为钋、铋和铅。

In summary, a radon atom in the air decays within a few days into its short-half-life radon daughters, which decay within about an hour; with these decays, three alpha particles are emitted, one by radon and two by its daughters.

Where does Radon come from?

Radon is a naturally occurring radioisotope. Radioactivity is, and always has been a part of the earth. Radon-222 is one of the elements in the long radioactive decay chain from uranium-238, and the less common isotope radon-220 is part of the decay series from thorium-232. The elements above radon in the chain are relatively long-lived and of less concern for radiation exposure, but radon and the elements immediately following it in the chain are short-lived and therefore more hazardous.

Whereas the predecessors to radon in the chain are solids and will not migrate far from their place in the soil, radon is a gas and can migrate through a few feet of earth. Cohen says that on the average, about six atoms of radon emerge form every square inch of soil every second. Radon in outside air is diluted rapidly, but if it enters through a basement floor and is trapped in a tight house, it can reach high concentrations.

From Lutgens & Tarbuck there is an estimate that the top 6 feet of soil from an average acre of land contains about 50 lbs of uranium. This corresponds to about 2 to 3 parts per million.

At what level is Radon dangerous?

It is generally assumed that no level of ionizing radiation is completely safe, but one must try to set a threshold of reasonably tolerable risk. The EPA suggests an "action level" of 4 picocuries per liter in air. Lutgens & Tarbuck suggest that this level corresponds ot about 8 to 9 atoms of radon decaying every minute in every liter of air. According to Cohen, the mean radon level in houses throughout the western world is about 1 pCi/l.

Cohen has expressed the risks of radiation in terms of "loss of life expectancy". He places the loss of life expectancy of 1 millirem of radiation at 1.2 minutes, and the loss of life expectancy of 20 hours for a years exposure to 1 pCi/l in your home. By this admittedly over-simplified model, you can project the equivalent exposure of 4 pCi/l for a full year as equivalent to 4000 millirem or 4 rem. The maximum permissible occupational exposure for persons working in radiation related occupations is 5 rem per year.

Other sources estimate the dose equivalent of 4 pCi/l to be as high as 14 rem, but I'm not clear on the rationale for that high figure.

Radon in the Air

One would think that radon was the least of our radiation problems since it is an inert gas. That would be so except that when we breathe, we are constantly passing air into our lungs and out of them. In this process, the radon gas simply goes in and out, doing little damage, but the radon daughters, being basically solid materials, and sometimes being electrically charged, can stick to the surfaces of our bronchial tubes. This puts them right where they can do the most harm, for the cells lining our bronchial tubes are among the cells of our body most sensitive to radiation-induced cancer. The alpha particles emitted in the decay of radon daughters, in spite of their poor penetrating power, can reach these very sensitive cells because they are deposited so close to them. To make matters very much worse, alpha particles are much more efficient than other types of radiation for inducing cancer. The very fact that they are not penetrating means that they dump a lot of their energy into each of the biological cells they pass through, and this large release of energy into a single cell is just what is needed to initiate a cancer. As a result an alpha particle is a hundred times more likely to cause cancer than other types of radiation, if it can reach the target cells. Our breathing processes allows the alpha particles from radon daughters to reach these cells.

Radon is believed to be an important cause of lung cancer, killing about 10,000 Americans each year. Only cigarette smoking causes more lung cancer deaths per year. And in perhaps one out of a thousand American homes, radon levels are so high they pose a greater lung cancer risk than smoking a pack of cigarettes per day.

More about the Watras home

A new nuclear power plant was preparing to start up near Pottstown, Pennsylvania, and its management began requiring workers to pass through a radiation monitoring portal to check whether any radioactive contamination was being carried out as they left the plant. Curiously, one of the workers, Stanley Watras, set off the alarm on entering the plant. The problem was traced to radon in his home. In fact, to this day his house still holds the world record, 2,700 pCi/l!

The Watras house is on the Reading Prong, a granite formation that extends from near Reading in southeastern Pennsylvania, through a wide band of northern New Jersey State (e.g., Morris County), a narrow band in New York State (e.g., Putnam County), and into Connecticut. This whole formation is known to have high uranium content and hence was expected to have radon problems. However, measurements now make it clear that the radon problems are largely confined to the Pennsylvania section plus extreme western New Jersey.

Radon compared to other radiation sources

The "action level" recommended by the Environmental Protection Agency for radon in the air is 4 picocuries/liter of air. It is difficult to convert air concentrations to actual exposures in rems or sieverts, but estimates are in the range of 4 to 14 rem per year at that concentration. That makes it greater that all the other routine environmental exposures combined.

The percentages for radon exposure and other sources in the pie chart at left were attributed to the EPA in an article by Charles Seabrook in The Atlanta Journal on July 10,1990. Seabrook says 40000 lung cancer deaths a year are blamed on radon. 15-20% of homes have unacceptably high levels (presumably meaning > 4 pCi/l). 左边饼图中氡暴露及其他来源的百分比归因于环保署（EPA），该信息出自1990年7月10日《亚特兰大先驱报》上查尔斯·塞布罗克的文章。塞布罗克称，每年有40000例肺癌死亡归因于氡。预计有15-20%的房屋存在不可接受的高水平（意味着>4 pCi/l）。